The invention relates to transient overvoltage suppressors, and more particularly to transient overvoltage suppressors which optimally combine metal oxide varistors, inductors, and silicon junction diode suppressors to achieve, at the lowest feasible cost, the lowest clamping level of a transient overvoltage having a very high open circuit voltage level and a very high short circuit current level.
The sudden failure or gradual step by step deterioration of electrical equipment due to high voltage transients or surges stemming from lighting or man-made occurrences has long been recognized as a serious problem in the electrical industry. The literature has reported many investigations of transient overvoltage occurrences on a wide variety of power distribution systems, both residential and commercial. These studies have shown that voltage spikes of 6 kilovolts or more are not uncommon on residential and commercial lines. Overvoltage transients generated by lighting or line switching or full voltage starting of electric motors can easily damage a wide variety of electrical and electronic equipment. Reliable protection of electrical equipment against such transient overvoltages by various surge protectors known in the prior art requires that these surge protectors be able to reduce the transient overvoltage to a safe value well below the maximum voltage rating of the protected electrical equipment. Recently, the Institute of Electrical and Electronics Engineers has issued a specification for the worst case overvoltage condition, as described in specification IEE-587 1980, that ordinarily will be expected on power mains within a building. This worst case overvoltage has an open circuit peak value of 6,000 volts and a short circuit peak current of 3,000 amperes. The open circuit voltage has a rise time of 1.5 microseconds to its crest, and a 50 microsecond delay to its half maximum value. The short circuit current has an 8 microsecond rise time and a 20 microsecond delay to fall to its half maximum value. Overvoltage surges of this type have very high energy, and can easily cause arcing within inadequately protected electrical equipment that will destroy power transistors, integrated circuits and other components, and can cause charring within printed circuit boards.
A variety of surge suppressor circuits intended to suppress such overvoltages have been proposed. The state of the art is generally indicated by U.S. Pat. Nos. 4,419,711; 3,793,535; 4,328,253; 4,212,045; 4,095,163; and 3,480,830. The prior art surge suppressor believed to be closest to the present invention includes a metal oxide varistor connected between the hot and neutral line conductors of the AC power line. An inductor is connected from the junction between the hot line conductor and the metal oxide varistor to the hot supply terminal to which the protected equipment is connected. A large silicon junction diode suppressor device, such as one marketed by the present assignee under the trademark "TRANSZORB", is connected between the hot supply terminal and the neutral line conductor. The inductor diverts a substantial amount of the surge current through the metal oxide varistor, reducing the amount of current that flows through the TRANSZORB suppressor in its breakdown mode. However, the degree of clamping that would be desired to provide adequate protection to the protected equipment against surges of the magnitude referred to in the above-mentioned IEEE specification at a satisfactorily low cost has not been accomplished. There is an unmet need for such a low cost surge suppressor.